With superchargers incorporated in internal combustion engines of automobiles, ships or vessels, for example, exhaust gases from an internal combustion engine are made use of rotating a turbine impeller on an exhaust side and a compressor impeller disposed on an intake side to be coaxial with the turbine impeller to take in an outside air to compress the same. Such superchargers function to supply an air as compressed to the internal combustion engine to achieve an improvement in output thereof.
Since the turbine impeller used in the supercharger described above is exposed to high-temperature exhaust gas discharged from the internal combustion engine, nickel alloys, alloys consisting of titanium and aluminum, etc., which are excellent in heat resisting strength, are ordinarily used therefor. On the other hand, since compressor impellers are made use of in those portions, in which an outside air is taken, and not exposed to high temperature, aluminum alloys, etc. are ordinarily used therefor.
Conventionally, aluminum alloys for compressor impellers include 354.0 (Al-9% Si-1.8% Cu-0.5% Mg alloy) and 355.0 (Al-5% Si-1.3% Cu-0.5% Mg alloy) specified in American Society for Testing and Materials Standard (ASTM), JIS-AC4C (Al-7% Si-0.3% Mg alloy), etc.
Also, for example, Prior Art Publication 1 discloses a high-pressure casting aluminum alloy containing, by mass %, 4 to 12% Si, 0.2 to 0.6% Mg, up to 0.3% Ti, and 0.001 to 0.01% B, another aluminum alloy containing 2 to 5% Cu further added to the composition of the above casting aluminum alloy, and a still another aluminum alloy containing 0.002 to 0.02% Sr further added to the compositions of the above aluminum alloys, respectively.
In recent years, various examinations have been made on high-speed rotation of turbine impellers and compressor impellers with a view to a further improving internal combustion engines in combustion efficiency. It is predicted in these examinations that with compressor impellers, an centrifugal force acting on an impeller upon rotation at high speed increases and an exposure temperature of around 150° C. in an actual situation increases to 180 to 200° C. due to high-speed rotation. Therefore, it is assumed that a further high strength in addition to a proper toughness at ordinary temperature is needed for compressor impellers, or it is assumed that a high strength is further needed at a temperature of 180 to 200° C.
Under the above background, it has been examined to apply, to materials of compressor impellers, magnesium alloys being higher in strength than conventional aluminum alloys, expensive titanium alloys being higher in strength than aluminum alloys and enabled to be made more light in weight than magnesium alloys, etc. Also, on the other hand, lightweight and inexpensive aluminum alloys are practically useful and an engineering development for making conventional aluminum alloys further high in strength is considerably expected. While aluminum alloys of high strength include, for example, an aluminum forged alloy A2618 (prescribed in ASTM), a characteristic comparable to that thereof is demanded of compressor impellers, for which a casting aluminum alloy is used.
Prior Art Publication 1: JP-A-6-145866